This invention relates to a novel composition and method for making electrostatically paintable elastomeric polyolefin articles by injection molding techniques for use as, for example, sight shields on the exterior of automotive vehicles.
Recently, use of elastomeric polyolefin polymers for decorative and functional automotive parts has taken on important commercial significance. The consumer, the government, and others are insisting on new industry standards combining safety, durability, performance and aesthetics, all at a minimum cost. Polyolefin elastomeric hydrocarbon copolymers are especially useful for automotive trim application, such as bumper assemblies for automobiles, because they are inexpensive, possess impact-absorbing properties and can be molded into many configurations. Further these elastomers have the ability to recover rapidly to their original state when deformed. However, in order to obtain an economically feasible process and product, it is necessary that the polyolefin composition is readily processable by injection molding and that the composition is electrostatically paintable.
More specifically, for automotive trim use a commercially acceptable elastomeric polyolefin shaped article must be paintable by conventional electrostatic techniques, especially the revolving atomizer process, such as disclosed in U.S. Pat. No. 2,698,814 dated Jan. 4, 1955, to assure economical use of paint and uniform coverage on hard-to-reach surfaces. When utilizing such painting techniques, the article containing carbon black should have a volume resistivity of less than 10.sup.8 ohm cm., as measured by the 2 by 5 inch method for testing of resistivity, described in Technical Report RG-128, Cabot Corporation, Boston, Mass. Conductivity of sufficient magnitude can be readily imparted to a polymer by compounding it with a sufficient level of carbon black. Some carbon blacks impart greater conductivity on an equal weight basis than others, consequently the amount of carbon black required to achieve a given level of conductivity will vary from one type of carbon black to another. Ease of processing during injection molding will vary with the amount and type of carbon black. In general, increasing the amount of carbon black decreases processing ease. Unfortunately, carbon blacks used heretofore do not impart sufficient conductivity to the polymer composition when used in such an amount as to retain easy processing characteristics, or they fail to yield the desired processing properties when used in the amounts necessary to give the goal conductivity, and others are so expensive as to make such articles commercially unacceptable. Therefore, there is a need in the art for an elastomeric polyolefin copolymer composition that is both readily processable by conventional injection molding techniques, electrostatically paintable, economical, and can be used as, for example, a sight shield placed between the bumper and fender on an automotive vehicle.